1. Field of the Invention
A variable output gas furnace has multistage gas burners that provide a variable heat output depending on need using variable fuel pressure and plural independently operable valves for the burners.
2. Description of Related Art
The turn down ratio of a furnace is the maximum firing rate divided by the lowest stable, clean firing rate at which a furnace can be operated. ##EQU1##
High turn down ratio capability is desirable in furnaces used in applications heating outside air to provide close control of the temperature of air leaving the furnace and entering an enclosure. As the outside air temperature increases and approaches the desired enclosure air temperature, less heat is required.
Presently commercial indirect gas fired duct furnaces are marketed by several manufacturers, and have a maximum turn down ratio of about 2.5 to 1. Therefore, the minimum firing rate is about 40% of the maximum firing rate. Good control of air temperature is achieved when outside air temperatures are low enough to be in the range of from 40% to 100% of the maximum firing rate. If the temperature difference between the desired supply air temperature and the outside air temperature is less than 40% of the furnace capability, the furnace must operate in a short cycle or rapid "on-off" mode which results in poor temperature control. When the furnace is "on", the air is overheated. When the furnace is "off", there is no heating. The "heat sink" characteristics of the furnace heat exchanger tend to reduce leaving air temperature oscillations, but uncomfortable air temperatures still often result.
The use of gas burners for heating air is common with various provisions made for adjusting the heat output for different seasons of the year and times of the day. F. Coyle, U.S. Pat. No. 948,133, issued Feb. 1, 1910, teaches plural burners in a single combustion box with independently operated burners and adjustable burn rates for each burner. W. Hartwig, U.S. Pat. No. 1,918,265, issued Jul. 18, 1933, teaches a plurality of burners operated by a thermostat and electromagnetic actuators with constantly available pilot or other ignition means. W. L. McGrath, U.S. Pat. No. 2,470,996, issued May 24, 1949, teaches plural burners with a common supply and individual burner control to vary the number of burners in operation and a main controller for varying the fuel flow and pressure in the supply conduit.